1. Field of the Invention
This invention relates to hearing aids. More specifically, it relates to digital hearing aids comprising two or more microphones in the audio signal path.
2. The Prior Art
Hearing aids with directional capabilities usually employ two or more microphones to permit the hearing aid to process incoming sounds according to direction in order to achieve increased sensitivity towards sound coming from a particular direction, or range of directions. In this process the hearing aid relies on differences in arrival time and sound level among the microphones. A hearing aid with a directional capability makes it easier for the hearing aid user to perceive a sound coming from a particular direction, as sounds from other directions are suppressed to some extent.
The term “directivity” is used throughout this application. This term signifies the capability of a hearing aid to favor sound originating from a particular direction or range of directions over sound originating from other directions. Physically, the definition of hearing aid directivity is the ratio between the output level due to sound from the favored direction and the output level due to sound averaged over the spherical integral from all directions, typically expressed in dB.
In order for a directional microphone system using omni-directional microphones to function to a reasonable degree of satisfaction it is necessary that the parameters of the individual microphones have been matched very closely to each other. The matching may be achieved in the production stage, e.g. by the careful selection of paired microphones, or, in the case of powerful digital processors, it may be achieved by adapting the processor to compensate for a difference in phase characteristics as measured individually with the particular set of microphones.
Directional microphone systems relying on arrival time differences must resolve minute differences in phase between the front and rear microphone signals in order to control the overall directional sensitivity of the combined front and rear microphone signals, especially at lower frequencies. A directional characteristic is in principle obtained by delaying the signal from the front microphone appropriately and subtracting the delayed microphone signal from the signal from the rear microphone. This requires that phase characteristics of the individual omni-directional microphones have been matched closely to each other.
From EP 1191817 A1 is known a hearing aid with adaptive microphone matching. This prior art hearing aid comprises means for comparing the signal levels from at least two microphones for the purpose of reducing the difference in the microphone signal levels. This matching only deals with differences in amplitude between the microphones, and does not take phase differences between the microphone signals into account.
US 2002/0034310 A1 describes a system for adaptively matching sensitivities of microphones in multi-microphone systems, e.g. in a directional hearing aid. The system utilizes a delay unit, a set of band-split filters, and means for scaling the microphone signals appropriately to match the sensitivities. This scaling is a band-level scaling at various frequencies only, and does not take phase differences into account.
From US 2004/0057593 A1 is known a hearing aid and a method for adaptive matching of microphones in the hearing aid. The method utilizes a feedback loop with a long time constant for matching the amplitude of the signal of the microphones. A fixed filter is used to match one of the microphones to the other microphone at manufacture, but means for changing the filter parameters at a later time are not incorporated. The matching of the microphones is not very accurate, and does not take phase variations into account.
EP 1458216 A2 describes an apparatus and a method for adapting microphones in hearing aids. The apparatus for performing microphone adaptation comprises a calibrated reference microphone, and the method of adapting the hearing aid microphone is carried out during manufacture of the hearing aid. The microphone adaptation described in EP 1 458 216 A2 does not take variations due to ageing of the microphones etc. into account.
If, during the service life of the hearing aid, the characteristics of the individual microphones change for some reason, e.g. ageing, temperature, humidity, or other factors, a matching of phase characteristics between the microphones provided in the production stage may no longer be accurate, with the potential result of a corruption of the directivity of the microphone system. This is, of course, an unacceptable situation and a need thus exists for a device or a method to keep the matching of the phase characteristics of the microphones within a certain tolerance throughout the service life of the hearing aid.
Known measures to prevent microphones from drifting over time include pre-ageing the microphones prior to assembly of the hearing aid in order to minimize drift over time during service life. Pre-ageing the microphones does not take the dependency of temperature, humidity or other environmental factors into account.
However, changes in the signal path that may occur over time cannot be taken into account. These changes in the signal path may, for example, originate from changes in temperature, humidity, component ageing, the replacement of one or both microphones by repair, etc.
If the microphones are selected among types of microphones with a frequency pole placed in the very low end of the frequency spectrum, e.g. 20-40 Hz, any differences in microphone poles essentially only affect the amplitude of the transfer function since any effects on the phase will only have effect at frequencies below the frequency range where the directional microphone system has to function.
Unfortunately, very low-frequency poles in a microphone mean that the microphone itself has a very high sensitivity in the vicinity of the pole, i.e. the range 20-40 Hz in the example in the foregoing. In a hearing aid, a high sensitivity to low frequencies in the microphones creates problems in many situations. Low frequencies are, for instance, not needed for conveying the perception of speech, and are thus in hearing aids considered unwanted signals. Low frequency noise sources nevertheless occur in many situations in modern society, e.g. when driving an automobile, or when exposed to wind noise in the outdoors. Microphones with a high sensitivity to low frequencies are easily brought into a state of saturation or acoustic overloading, wherein the microphone diaphragm itself reaches the limits of its suspension by the movements inflicted by the low frequency air pressure variations. When saturated, the microphone is prohibited from conveying sound efficiently, and a listener gets the impression that the sound has been suddenly cut off, or at least severely distorted.
Microphones having less sensitivity to low frequencies are thus to be preferred in hearing aids. However, this means poles at somewhat higher frequencies, and thereby rising importance of an accurate matching of phase characteristics.
The prior art methods of matching are either not sufficiently accurate, or they are unfit for matching any microphones but those having low-frequency poles. If microphones having less sensitivity to low frequencies—and thus poles placed higher in the frequency spectrum—are to be used, a more effective approach to matching the microphones is needed. This approach should preferably be independent of the placement of the poles in a given set of microphones, and thus freely allow matching of arbitrary microphones including those with poles at higher frequencies.
The system consisting of the microphone and the subsequent RC filter stage may be modeled with one of several approaches. The transfer function of the model may comprise only the most dominant pole of the system, resulting in a simple first order model, or it may take into account both the pole of the microphone itself and the pole of the RC filter stage, resulting in a more complex second order model. Utilizing a second-order model incorporating both the microphone and the RC filter stage complicates the matching process somewhat because a second-order system is more complicated, and thus takes more resources to model. On the other hand, it offers the prospect of a more refined matching, and allows an additional degree of freedom in the selection of microphones to be incorporated into the system.
To address the problem of achieving an accurate matching of both the amplitude and the phase of the microphones, an adaptive matching during use of the microphones, or ideally of the entire analog part of the signal path, must be made. This may be achieved by using an accurate matching system matching the microphones during use.